1. Field of the Invention
The invention relates to apparatus for urging a device against a wall of a hole and particularly to a mechanism for extending an arm and fixing the device at a desired depth within a bore hole.
2. Description of the Prior Art
In logging a well, a special tool known as a sonde is lowered into the well bore hole on a conductive armored cable. Typically the sonde is lowered to the bottom of the bore hole and raised slowly during the logging process. Occasionally the sonde is lowered to a selected depth of interest during a sampling period. To maintain the depth of the sonde, and to urge the tool against the bore hole wall for obtaining the best signal, a mechanical clamping mechanism may extend from the tool to engage the wall of the bore hole.
Several clamping mechanisms have been developed to fix a sonde within a bore hole. One mechanism previously used consists of an extension arm pivotally coupled to the sonde. A leaf spring or other biasing mechanism positioned between the sonde and the arm is used to extend the arm and force the arm away from the sonde, forcing both the sonde and the arm against the bore hole wall. Illustrative of such a mechanism is U.S. Pat. Nos. 4,614,250 and 4,575,831, both assigned to Schlumberger Technology Corporation. Several disadvantages occur with the biasing means for extending the arm. First, the force may not be sufficient to fix the sonde in place with slack in the cable. A second disadvantage may be in that the extension distance of the arm is not controllable. That is to say there is no in between distance. The arm must extend as far as the bore hole will allow.
Another mechanism often used to extend a sonde arm is a hydraulic jack. The hydraulic mechanism may be used for anchoring as well as for up-hole operations when extension of a pad or a portion of the sonde must be in contact with the bore hole wall at all times. Illustrative of a hydraulic jack extension mechanism are U.S. Pat. Nos. 4,751,688 and 4,715,470 assigned to Chevron Research Company. A disadvantage with the hydraulic jack or piston extension mechanism is the need for a fluid reservoir and pumping device. In addition, the diameter of the sonde limits the extension distance since the extension axis is generally perpendicular to the longitudinal axis of the sonde.
Yet another mechanism used to extend a clamping pad or arm may be by a linear drive mechanism. Such a mechanism is illustrated in U.S. Pat. No. 4,715,469, assigned to Petrophysical Services, Inc., a division of Western Atlas International, Inc. and the assignee of this invention. The drive mechanism may be comprised of a motor which powers a threaded shaft which retains a ball nut or truck. One end of a push rod is pivotally coupled to the ball nut. The opposite end of the push rod is pivotally coupled to the extension arm. The electric motor is reversible therefore allowing extension and retraction of the arm. The extension mechanism provides its greatest force towards the point of greatest extension. With the arm in the fully retracted position, the push rod and the extension arm are nearly parallel. In extending the arm from the closed position, the force applied by the linkage is along the length of the arm with little or no lateral force to drive the arm away from the sonde. Occasionally the clamping or extension arm may not extend because the force from the drive gear is parallel to the longitudinal axis of the arm. That is there is no lateral component to move the arm away from the sonde. In another instance, the lateral force may push the arm in towards the tool. In each case, the arm does not move away from the tool and a strain is placed upon the drive mechanism as well as on the connections between the components, leading to early fatigue and possible failure. Another disadvantage occurs in situations where the tool is used in a deviated hole and the weight of the tool is upon the arm in the closed position. The drive mechanism may be unable to produce sufficient force to open the arm and lock the tool in place in the bore hole.